Competition for inorganic and organic forms of nitrogen and phosphorous between phytoplankton and bacteria during an Emiliania huxleyi spring bloom

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Løvdal, T. ; Eichner, C. ; Grossart, H.-P. ; Carbonnel, V. ; Chou, L. ; Martin-Jézéquel, V. ; Thingstad, T. F. (2008)

Using <sup>15</sup>N and <sup>33</sup>P, we measured the turnover of organic and inorganic nitrogen (N) and phosphorus (P) substrates, and the partitioning of N and P from these sources into two size fractions of marine osmotrophs during the course of a phytoplankton bloom in a nutrient manipulated mesocosm. The larger size fraction (&gt;0.8 μm), mainly consisting of the coccolithophorid <i>Emiliania huxleyi</i>, but also including an increasing amount of large particle-associated bacteria as the bloom proceeded, dominated uptake of the inorganic forms NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>&minus;</sup>, and PO<sub>4</sub><sup>3&minus;</sup>. The uptake of N from leucine, and P from ATP and dissolved DNA, was initially dominated by the 0.8&ndash;0.2 μm size fraction, but shifted towards dominance by the &gt;0.8 μm size fraction as the system turned to an increasing degree of N-deficiency. Normalizing uptake to biomass of phytoplankton and heterotrophic bacteria revealed that organisms in the 0.8&ndash;0.2 μm size fraction had higher specific affinity for leucine-N than those in the &gt;0.8 μm size fraction when N was deficient, whereas the opposite was the case for NH<sub>4</sub><sup>+</sup>. There was no such difference regarding the specific affinity for P substrates. Since heterotrophic bacteria seem to acquire N from organic compounds like leucine more efficiently than phytoplankton, our results suggest different structuring of the microbial food chain in N-limited relative to P-limited environments.
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